I wear my black pea coat with brown brogue boots, skinny indigo jeans and a brown belt. No one has ever remarked that the colours don't go (my female flatmates do have a habit of letting me know when I slip up).

However, I would veer away from wearing black, brown and navy in a formal outfit. If your strapped for cash there's a rather lovely camel trench coat in H and M now at the moment for only about 60 something quid. Would look great with a navy suit.